This disclosure demonstrates that single atomic layer MoS2 can be used in chemical vapor sensors and that the response is selective to analytes that are strong electron donors and some polar molecules.
This disclosure demonstrates that the response of the MoS2 chemical sensors nicely complements the response to sensors fabricated from graphene and carbon nanotube (CNT) meshes. In particular, when combined, the three materials can provide a sensing suite that can correctly identify triethylamine (TEA) and acetone vapors.
Furthermore, this disclosure concerns a new type of chemical sensor created from a variety of low-dimensional materials including but not limited to graphene, carbon nanotubes, and monolayer forms of a variety of transition metal dichalcogenides that will be able to accurately identify with great sensitivity and precision a variety of airborne chemicals of interest.
The planar habit of two-dimensional (2D) materials is attractive for the ultimate size scaling of many types of devices, offers relative ease of fabrication, the requisite large-scale integration, and exceedingly low power consumption.
The very high surface-to-volume ratio of such single or few monolayer materials enables highly efficient gating of charge transport via surface gates, obviating the need for the more complex growth and fabrication procedures, and offers an enormous functional area per volume for chemical sensing applications.
Graphene has captivated the attention of the scientific community since the first measurements of high mobility transport were reported in single layer flakes. Recent effort has focused on other 2D materials such as the transition metal dichalcogenides, and field effect transistors with a monolayer of MoS2 as the active channel were shown to exhibit high on/off ratios at room temperature, ultra-low standby power dissipation and well-defined photoresponse.
The high surface-to-volume ratio is also important for new sensor materials, which must exhibit selective reactivity upon exposure to a range of analytes (determined by the character of surface physisorption sites), rapid response and recovery, and sensitive transduction of the perturbation to the output parameter measured.
The conductivity of graphene near the charge neutrality point has been shown to change with adsorption of a variety of analytes, but annealing to 150° C. was required to restore the conductivity to its original value, suggesting the analytes were strongly bound. Other work has shown that graphene's intrinsic response to physisorption of analytes such as ammonia is very small. The sensitivity can be enhanced by functionalizing the graphene surface, e.g. by oxidation, but these devices showed little selectivity, and functionalization introduces additional complexity to the fabrication process.
Recent work has shown that measuring analyte-dependent changes in the low frequency noise spectrum can enhance the selectivity of graphene sensors, although degassing in vacuum at room temperature for several hours between measurements was needed to obtain good reproducibility. Other 2D materials are likely to offer selective surface reactivity to physisorbed species, and if semiconducting in character, can provide both lower background carrier densities and the possibility of photo-modulated sensing mechanisms. Furthermore, in the past it has been shown that 1-dimensional (1D) CNTs are quite responsive to a variety of analytes, although their response is also not selective.
Due to the wide variety of available low-dimensional materials, each having its own unique chemistry and electronic properties, we designed a new class of full-spectrum chemical sensors that are able to selectively and sensitively respond to a wide variety of analytes by combining the sensing properties of each low-dimensional material in parallel.
By combining the responses from each low-dimensional material, we can positively identify chemicals of interest with a never-before-seen accuracy.
Furthermore, because these materials are atomic-scaled (for instance, graphene is ˜3 Angstroms thick, MoS2 is ˜6 Angstroms thick), these full-spectrum sensor suites can be integrated into applications demanding the smallest of space restrictions, and address system integration issues. Additionally, as two terminal low-dimensional devices require very low currents (˜nA) for successful operation, these sensor suites can be integrated into applications demanding ultra-low power electronics. Moreover, due to the atomic size and enormous surface to volume ratio of the low-dimensional films, any small adsorbate will cause an immediate electrical response, making these sensors incredibly fast to react to vapor analytes. The combination of sensitivity, versatility, atomic size, low-power, high reaction speed, and low cost of fabrication make these low-dimensional sensing suites superior to any other chemical vapor sensor currently on the market, which usually emphasizes only one or two of these advantageous qualities.